Monopole tip for ablation catheter and methods for using same

ABSTRACT

An ablation catheter apparatus with a monopole antenna that is arranged to provide a relatively uniform electric field and a method for using such an ablation catheter apparatus are disclosed. According to one aspect of the present invention, an ablation catheter includes an elongated flexible tubular member that is adapted to be inserted into the body of a patient, and a transmission line that is disposed within the tubular member. The transmission line has a distal end and a proximal end which is arranged to be connected to an electromagnetic energy source. The catheter also includes a monopole antenna with tip section and a body section that includes a distal end and a proximal end. The tip section and the body section are arranged to produce a relatively uniform electric field around the monopole antenna which is sufficiently strong to cause tissue ablation. The proximal end of the body section of the monopole antenna is arranged to be electrically coupled to the transmission line.

[0001] This application is a Continuation of U.S. patent applicationSer. No. 09/321,666, filed May 28, 1999, which is incorporated herein byreference, in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates generally to ablation cathetersystems that use electromagnetic energy in the microwave frequency rangeto ablate internal bodily tissues. More particularly, the presentinvention relates to a monopole tip for a catheter that enables distalfire capabilities while enabling a relatively even electromagnetic fieldto be created at the sides of the monopole tip to facilitate theablation of cardiac tissue.

[0004] 2. Description of the Related Art

[0005] Catheter ablation is a therapy that is becoming more widely usedfor the treatment of medical problems such as cardiac arrhythmias,cardiac disrhythmias, and tachycardia. Most presently approved ablationcatheter systems utilize radio frequency (RF) energy as the ablatingenergy source. However, RF energy has several limitations which includethe rapid dissipation of energy in surface tissues. This rapiddissipation of energy often results in shallow “burns,” as well as afailure to access deeper arrhythmic tissues. As such, catheters whichutilize electromagnetic energy in the microwave frequency range as theablation energy source are currently being developed. Microwavefrequency energy has long been recognized as an effective energy sourcefor heating biological tissues and has seen use in such hyperthermiaapplications as cancer treatment and the preheating of blood prior toinfusions. Catheters which utilize microwave energy have been observedto be capable of generating substantially larger lesions than thosegenerated by RF catheters, which greatly simplifies the actual ablationprocedures. Some catheter systems which utilize microwave energy aredescribed in the U.S. Pat. No. 4,641,649 to Walinsky; U.S. Pat. No.5,246,438 to Langberg; U.S. Pat. No. 5,405,346 to Grundy, et al.; andU.S. Pat. No. 5,314,466 to Stern, et al., each of which is incorporatedherein by reference in its entirety.

[0006] Cardiac arrhythmias, which may be treated using catheterablation, are generally circuits, known as “reentry circuits,” whichform within the chambers of the heart. As is known to those skilled inthe art, reentry circuits are abnormal electrical pathways that may formin various areas of the heart. For example, reentry circuits may formaround veins and/or arteries which lead away from and to the heart.Cardiac arrhythmias may occur in any area of the heart where reentrycircuits are formed.

[0007] The catheters used for treatment of cardiac arrhythmias,disrhythmias, and tachycardia may have a variety of different antennaconfigurations to create electromagnetic fields used in ablation. Somecatheters have antennas that essentially protrude from the distal endsof the catheters. In other words, some catheters have antennas whichform the distal tips of the catheters. A monopole antenna is typicallyconfigured to form the distal tip of a catheter.

[0008]FIG. 1a is a diagrammatic representation of a distal end of acatheter with a monopole antenna at its tip. A distal end 102 of acatheter has a monopole antenna 108 at its tip. As shown, monopoleantenna 108 has a rounded shape, and is coupled to a center conductor112 of a co-axial transmission line 116. Typically, monopole antenna 108is formed from a metallic material. Distal end 102 of the catheter mayalso include electrodes 120, which may be used for mapping processes,that may be coupled to processing equipment (not shown) using ECG wires122.

[0009] Monopole antenna 108 is often arranged to be used in ablatingtissue. Center conductor 112 transmits energy, e.g., electromagneticenergy, to monopole antenna 108 to allow an electromagnetic field to beformed with respect to monopole antenna. FIG. 1b is a diagrammaticrepresentation of a monopole antenna, i.e., monopole antenna 108 of FIG.1 a, shown with electromagnetic field lines. Electromagnetic field lines130 generally radiate from monopole antenna 108 in a substantiallyellipsoidal pattern. Hence, near sides 134, “hot spots” 138 ofelectromagnetic energy are typically formed. Hot spots 138 are generallyassociated with the highest amounts of electromagnetic energy radiatedby monopole antenna 108. The existence of hot spots 138 causes certainportions of a myocardium of heart, for example, such as those that aresubstantially contacted by a hot spot to be ablated more than otherportions.

[0010] When an ablation procedure is performed using monopole antenna108, the depth of cuts formed may not be uniform, since electromagneticfield lines 130 are not uniform. That is, the shape, or profile, ofelectromagnetic field lines 130 are such that when ablation isperformed, the depth associated with the ablation may not be even. Thelack of even depth in an ablation procedure may cause the ablation,e.g., an ablation in the myocardium of a heart, to be unsuccessful, asall of the cardiac tissue may not be effectively ablated. Hence, theablation procedure may have to be repeated, which is both time-consumingand inefficient.

[0011] Therefore, what is needed is a monopole antenna structure for usewith an ablation catheter that efficiently allows tissue to be ablated.More specifically, what is desired is a monopole antenna structure thatis capable of producing a relatively field, e.g., electromagnetic field,a deep lesion, and a microwave power deposition at the tip of acatheter, i.e., a tip-firing catheter.

SUMMARY OF THE INVENTION

[0012] The present invention relates generally to an ablation catheterwith a monopole antenna that is arranged to provide an electric fieldthat is able to produce a deep lesion, e.g., in the myocardium or aheart, and has a tip-firing capability. According to one aspect of thepresent invention, an ablation catheter includes an elongated flexibletubular member that is adapted to be inserted into the body of apatient, and a transmission line that is disposed within the tubularmember. The transmission line has a distal end and a proximal end whichis arranged to be connected to an electromagnetic energy source. Thecatheter also includes a monopole antenna with tip section and a bodysection that includes a distal end and a proximal end. The tip sectionand the body section are arranged to produce a relatively uniformelectric field around the monopole antenna which is sufficiently strongto cause deep tissue ablation. The proximal end of the body section ofthe monopole antenna is arranged to be electrically coupled to thetransmission line.

[0013] In one embodiment, the transmission line is a coaxial cable,which has a center conductor and an outer conductor. In such anembodiment, the proximal end of the monopole antenna is arranged to beelectrically coupled to the center conductor. In another embodiment, thebody section of the monopole antenna is tapered such that the diameterat the proximal end of the body section of the monopole antenna issmaller than the diameter at the distal end of the body section of themonopole antenna.

[0014] According to another aspect of the present invention, an antennastructure arranged to be used in an ablation catheter has a longitudinalaxis, and includes a body section with a first end and a second end, atip section, and a transition section. The body section is sized suchthat the axial cross-sectional area about the longitudinal axis of thesecond end is smaller than the axial cross-sectional area about thelongitudinal axis of the first end. The second end is arranged to beelectrically coupled to a transmission line, and the body section isshaped to allow a relatively uniform electric field to be formed withrespect to the antenna structure.

[0015] The tip section has a proximal portion that has an axialcross-sectional area about the longitudinal axis which is greater thanor approximately equal to the axial cross-sectional area of the firstend, and the transition section is disposed between the proximal portionand the first end.

[0016] In one embodiment, the first end has a diameter that is greaterthan the diameter of the second end, and the proximal portion has adiameter that is greater than or equal to the diameter of the first end.In such an embodiment, the tip section may have a diameter that is lessthan the diameter of the first end.

[0017] In accordance with still another aspect of the present invention,a microwave ablation catheter includes an elongated flexible tubularmember, which has a distal portion, a proximal portion, and alongitudinal catheter axis, and is adapted to be inserted into a vesselin the body of a patient. The microwave ablation catheter also includesa transmission line with a proximal end and a distal end. Thetransmission line is disposed within the tubular member, and theproximal end of the transmission line is suitable for connection to anelectromagnetic energy source. A monopole antenna which is part of themicrowave ablation catheter is coupled to the transmission line forgenerating an electric field sufficiently strong to cause tissueablation, and includes a frusto-conically shaped emitting surface withan axis that is substantially parallel to the longitudinal catheteraxis. In one embodiment, the monopole antenna further includes a roundeddistal emitter surface. In such an embodiment, the antenna may alsoinclude a trough region between the frusto-conically shaped emittingsurface and the distal emitter surface, as well as an encapsulatingmaterial that encapsulates the trough and frusto-conically shapedemitting surface such that the trough forms an anchor for theencapsulating material.

[0018] These and other advantages of the present invention will becomeapparent upon reading the following detailed descriptions and studyingthe various figures of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

[0019] The invention may best be understood by reference to thefollowing description taken in conjunction with the accompanyingdrawings in which:

[0020]FIG. 1a is a diagrammatic representation of a distal end of acatheter with a monopole tip.

[0021]FIG. 1b is a diagrammatic representation of a monopole antenna,i.e., monopole antenna 108 of FIG. 1a, shown with electromagnetic fieldlines.

[0022]FIG. 2a is a diagrammatic representation of an ablation catheterin accordance with an embodiment of the present invention.

[0023]FIG. 2b is a perspective representation of a monopole antenna witha tapered configuration, i.e., monopole antenna 202 of FIG. 2a, inaccordance with an embodiment of the present invention.

[0024]FIG. 3a is a diagrammatic side view representation of a monopoleantenna, shown with a contour plot of the magnitude of electric fieldlines, in accordance with an embodiment of the present invention.

[0025]FIG. 3b is a diagrammatic side view representation of a monopoleantenna, i.e., monopole antenna 302 of FIG. 3a, shown with relativespecific absorption rates, in accordance with an embodiment of thepresent invention.

[0026]FIG. 4 is a diagrammatic cross-sectional representation of adistal end of a catheter which includes a monopole antenna in accordancewith an embodiment of the present invention.

BRIEF DESCRIPTION OF THE EMBODIMENTS

[0027] When the electromagnetic field associated with an antenna in anablation catheter is not uniform, the depth of an ablation formed incardiac tissue using the catheter is often uneven. Ablation catheterswith conventional monopole antennas generally do not emit uniformelectric fields. Instead, the contour of electric field lines, as wellas hot spots in the electric field around a monopole antenna, are suchthat ablation of cardiac tissue, as for example in a myocardium of aheart, are often uneven. As a result, the ablation of the tissue may notbe successful.

[0028] An ablation catheter that has a monopole antenna which is shapedto enable a substantially uniform field, e.g., electromagnetic orelectric field, to be formed around the monopole antenna allows thedepth of an ablation of tissue to occur substantially uniformly Inaddition, such a monopole antenna allows the catheter to have forwardfiring, or tip-firing, capabilities. That is, the distal tip of themonopole antenna may also be used to ablate tissue.

[0029] When the depth of an ablation is relatively uniform, i.e., has asubstantially uniform depth, an overall ablation process may be moreefficiently performed, as it may be unnecessary to repeatedly ablate thesame area of tissue to obtain an even depth of ablation. When an overallablation process is more efficient, in that the time spent performingablation may be reduced.

[0030] A monopole antenna which includes a tip section and a taperedbody section enables hot spots in the electromagnetic field formedaround the body section to be substantially eliminated. FIG. 2a is adiagrammatic representation of an ablation catheter with a monopoleantenna, which includes a tip section and a tapered body section, inaccordance with an embodiment of the present invention. An ablationcatheter 180, which is suitable for use as a microwave ablationcatheter, is generally arranged to be introduced into the body of apatient through a blood vessel, e.g., the femoral vein. Catheter 180 maybe considered to be an overall elongated, flexible, tube. It should beappreciated that for ease of illustration, catheter 180 has not beendrawn to scale.

[0031] Since catheter 180 is arranged to be used within the body of apatient, materials used to form catheter 180 are typically biocompatiblematerials. Suitable biocompatible materials used to form catheter 180include, but are not limited to medical grade polyolefins,fluoropolymers, polyurethane, polyethylene, or polyvinylidene fluoride.In one embodiment, a PEBAX resin, which is available commercially fromElf Atochem of Germany, may be used in the formation of catheter 180.

[0032] Catheter 180 includes a monopole antenna 202 from which anelectric field may be emitted to cause ablation. As shown, monopoleantenna 202 is located at the distal end of catheter 180. Monopoleantenna 202, which may be machined from a material such as stainlesssteel using a mill or a lathe, will be discussed below with reference toFIG. 2b. Typically, once catheter 180 is introduced into the body of apatient, catheter 180 is manipulated through a blood vessel and into theheart such that monopole antenna 202 may be positioned within a cardiacchamber in which an ablation procedure is to be performed.

[0033] Catheter 180 also includes electrodes 204 which are positioned oncatheter 180 such that they are located proximally with respect tomonopole antenna 202. Electrodes 204 are generally arranged to detectelectro-physiological signals from cardiac tissue. Hence, electrodes204, which are generally electrode bands, may be used to map therelevant region of the heart, i.e., the portion of the heart with whichan ablation procedure is associated, prior to or after an ablationprocedure. Electrodes 204 may also be used to aid in positioningcatheter 180 during an ablation procedure. In general, althoughelectrodes 204 may be formed from any suitable material which hasbiocompatible characteristics, electrodes 204 are typically formed frommaterials which include, but are not limited to, stainless steel andiridium platinum.

[0034] A handle 205 is often located near a proximal end of catheter180, although it should be appreciated that handle 205 is notnecessarily included as a part of catheter 180. Handle 205 is arrangedto enable a user, i.e., an individual who is performing an ablationprocedure on a patient, to grip and to manipulate catheter 180. In thedescribed embodiment, a connector 206 is located on catheter 180 suchthat connector 206 is proximal to handle 205. Connector 206 is arrangedto couple a transmission line (not shown), which is located withincatheter 180, to a power supply, or similar device, that is designed togenerate controlled electromagnetic energy.

[0035] As mentioned above, monopole antenna 202 is arranged to providean electric field, e.g., an electromagnetic field, to allow tissue to beablated. In the described embodiment, monopole antenna 202 is shapedsuch that the electric field which is generated is effectively confinedto the monopole region associated with monopole antenna 202. Withreference to FIG. 2b, a monopole antenna with a tapered body sectionwill be described in accordance with an embodiment of the presentinvention. FIG. 2b is a perspective representation of monopole antenna202 of FIG. 2a. Monopole antenna 202 includes a body section 208, anintermediate section 210, and a tip section 214. In the describedembodiment, body section 208 has a tapered shape, e.g., body section 208is shaped substantially as a conical structure with no single apexpoint. That is, body section 208, which includes an emitting surface,may have a frusto-conical shape. A proximal end 218 of body section 208generally has the smallest axial cross-sectional area, about alongitudinal axis of monopole antenna 202, associated with body section208. By way of example, the diameter of proximal end 218, about thelongitudinal axis of monopole antenna 202, is typically smaller than anyother diameter, along the same axis, that is associated with bodysection 208.

[0036] Intermediate section 210 effectively separates body section 208from tip section 214. One purpose of intermediate, or “trough,” section210 is to allow a material which is used to encase body section 208 tobe anchored with respect to monopole antenna 202. In other words,intermediate section 210 is shaped such that a material whicheffectively encapsulates body section 208 and, further, at least part ofintermediate section 210, is generally prevented from “peeling away”from intermediate section 210 and body section 208. The encapsulatingmaterial serves as a plug that holds monopole antenna 202 against acatheter, e.g., catheter 180 of FIG. 2a. In general, any suitablematerial may be used to form a plug that essentially encases bodysection 208. Such materials include, but are not limited to, Teflon,such as PolyTetraFluoroEthylene (PTFE), and Polyethylene (PE).

[0037] As shown, intermediate section 210 has an axial cross-sectionalarea that is less than the largest axial cross-sectional area associatedwith body section 208, i.e., the axial cross-sectional area associatedwith a distal end 222 of body section 208. In one embodiment, sinceintermediate section 210 and body section 208 have substantiallycircular cross-sectional areas, the diameter of intermediate section 210is less than the diameter of distal end 222 of body section 208.

[0038] Tip section 214 typically includes a distal portion 214 a and aproximal portion 214 b. Distal portion 214 a generally has a roundedshape. In the described embodiment, distal portion 214 a has anapproximately hemispherical shape. Proximal portion 214 b has asubstantially cylindrical shape, although it should be appreciated thatthe shape of proximal portion 214 b may vary widely. In someembodiments, tip section 214 may include only distal portion 214 a.

[0039] Generally, the dimensions associated with monopole antenna 202may vary, depending upon the overall configuration of a catheter inwhich monopole antenna 202 is used. By way of example, the dimensionsmay vary in order to achieve electric field lines of a particular shape.Typically, body section 208 has a longitudinal length in the range ofapproximately 0.25 inches to approximately 0.4 inches, e.g.,approximately 0.3 inches. The longitudinal length of intermediatesection 210 may range from approximately 0.07 inches to approximately0.10 inches, e.g., the longitudinal length of intermediate section 210may be approximately 0.09 inches. Finally, the longitudinal length oftip section 214 may range from total length of approximately 0.08 inchesto approximately 0.1 inches. In one embodiment, distal portion 214 a oftip section 214 may have a longitudinal length of approximately 0.06inches.

[0040] In addition to having a longitudinal length that may vary,monopole antenna 202 has diameters that may also be widely varied. Asdiscussed above, body section 208 may have a tapered shape, e.g., afrusto-conical shape. Accordingly, the diameters along the longitudinalaxis of body section 208 will generally vary. For example, the proximalend 218 of body section 208 may have a diameter which ranges betweenapproximately 0.025 inches to approximately 0.04 inches, while thedistal end 222 of body section 208 may have a diameter which ranges fromapproximately 0.06 inches to approximately 0.08 inches. It should beappreciated that the ranges of diameters may vary widely depending uponthe requirements of an overall catheter system.

[0041] The diameter of intermediate section 210 may also be widelyvaried. In general, the diameter of intermediate section 210 may be anysuitable diameter that is less than or equal to the diameter of distalend 222 of body section 208. However, the diameter of intermediatesection 210 is preferably less than the diameter of distal end 222 ofbody section 208, in order for a plug to be securely formed around bodysection 208, as previously mentioned. By way of example, when distal end222 of body section 208 has a diameter which ranges betweenapproximately 0.6 inches and approximately 0.8 inches, then intermediatesection 210 may have a diameter which ranges between approximately 0.04inches to approximately 0.06 inches.

[0042] Like the other diameters associated with monopole antenna 202,the diameter associated with tip section 214 may also vary. In thedescribed embodiment, the diameter associated with proximal portion 214b is substantially the same as a diameter associated with distal portion214 a. That is, when proximal portion 214 b is approximately cylindricalin shape, and distal portion 214 a is substantially hemispherical inshape, the diameters of proximal portion 214 b and distal portion 214 amay be approximately the same. For instance, the diameters may be in therange of approximately 0.08 inches to approximately 0.1 inches, althoughit should be understood that the diameters may be widely varied.

[0043] A monopole antenna such as monopole antenna 202 may be formedfrom substantially any conductive material. In general, monopoleantennas are preferably formed from materials with relatively highconductivity characteristics. Since catheters which include monopoleantennas are typically arranged to be inserted into human bodies, themonopole antennas are further formed from biocompatible materials, orare coated with a conductive biocompatible material, e.g., silver orplatinum.

[0044] Monopole antenna 202, as mentioned above, is shaped to enable asubstantially elliptical electromagnetic field to be formed aroundantenna 202. FIG. 3a is a diagrammatic side view representation of amonopole antenna, shown with contour lines associated with the magnitudeof an associated electric field, in accordance with an embodiment of thepresent invention. Contour lines 304 are shown with respect to fieldpropagation at ninety degrees of a cycle. As will be appreciated bythose skilled in the art, a cycle is a phase shift of 360 degrees. Thenumber of cycles per second will generally vary depending upon thefrequency that is being used, which often varies depending upon theneeds of a particular system. By way of example, in one embodiment, at afrequency of approximately 2.45 GigaHertz (GHz), the number of cyclesper second is approximately 2.45×10^(9.)

[0045] For purposes of illustration, representative contour lines 304 ofthe magnitude of an electric field have been shown, although it shouldbe appreciated that many more contour lines 304 associated with themagnitude of an electric field will generally exist. The magnitude of anelectric field generally varies with the distance from monopole antenna202. Specifically, the magnitude of an electric field decreases as thedistance from monopole antenna 202 increases. For example, the magnitudeof the portion of the electric field represented by contour line 304 ais greater than the magnitude of the portion of the electric fieldrepresented by contour line 304 c. In the described embodiment, theoutput power associated with monopole antenna 202 is approximately oneWatt (W), and the magnitude of the electric field represented by contourline 304 a is approximately 1000 Volts per meter (V/m). In such anembodiment, the magnitude of electric field line 304 c may beapproximately 500 V/m.

[0046] Ablation procedures that are performed with monopole antenna 202may be more efficient than those performed using a conventional monopoleantenna, in that the ablation of tissue is generally more even, e.g.,the depth of an ablation made in cardiac tissue may be uniform.Specifically, the tip-firing capabilities of monopole antenna 202, aswell as the deep penetration of the energy which emanates from monopoleantenna 202, may allow for a more efficient treatment of flutters andtachycardias, for example.

[0047] Monopole antenna 202 has an associated specific absorption rate(SAR), as will be understood by those skilled in the art. FIG. 3b is adiagrammatic side view representation of a monopole antenna, i.e.,monopole antenna 302 of FIG. 3a, shown with a pattern specificabsorption rates, in accordance with an embodiment of the presentinvention. The specific absorption rate associated with an antenna maybe expressed as follows: ${SAR} = \frac{\sigma \quad E^{2}}{2}$

[0048] where σ is the associated electrical conductivity at a particularfrequency, e.g., approximately 2.45 GHz, and E² is the square of themagnitude of the electric field. As the magnitude of the electric fieldvaries with distance from monopole antenna 202, the specific absorptionrate also varies. Since the specific absorption rate is a function ofthe magnitude of the electric field, the specific absorption ratedecreases as the distance from monopole antenna 202 increases.

[0049] In the described embodiment, specific absorption rate 354 a isthe highest rate associated with monopole antenna 202, while specificabsorption rate 354 c is the lowest rate associated with monopoleantenna 202. The pattern of specific absorption rates have been shown asincluding three rates 354, it should be appreciated that more ratesgenerally exist although, in some embodiments, fewer rates may be inexistence.

[0050]FIG. 4 is a diagrammatic cross-sectional representation of adistal end of a catheter which includes a monopole antenna in accordancewith an embodiment of the present invention. A distal end 400 of acatheter includes a monopole antenna 402 which has a tapered bodysection 408, an intermediate section 410, and a tip section 414. Forillustrative purposes, distal end 400 of catheter has not been drawn toscale. In the embodiment as shown, monopole antenna 402 also includes asurface finish 418, or coating, that covers the exterior of tip section414. Surface finish 418 may be formed from a variety of differentmaterials.

[0051] By way of example, surface finish 418 may be a silver plating. Itshould be appreciated that in another embodiment, monopole antenna 402may not include a surface finish.

[0052] In the described embodiment, monopole antenna 402 is coupled toan electromagnetic wave generator that is external to the catheter (notshown) through a coaxial cable 430. Specifically, a center conductor 432is electrically coupled to a proximal end of body section 408. As shown,body section 408 is bored out, e.g., includes a proximal bore 409, thatis arranged to allow center conductor 432 to be electrically coupled tomonopole antenna 402. In order to facilitate coupling of centerconductor 432 to body section 408, center conductor 432 extends past anouter conductor 436, or a shield, of coaxial cable 430. A variety ofdifferent methods may be used to couple center conductor 432 to bodysection 408. By way of example, center conductor 432 may be coupled tobody section 408 using a crimping process. An inner dielectric 434 ofcoaxial cable 430 serves to separate center conductor 432, which isarranged to carry electrical current, from shield 436 of coaxial cable430. As will be appreciated by those skilled in the art, outer conductor436 is often used for grounding purposes. Although coaxial cable 430 isarranged to provide power to monopole antenna 402, it should beappreciated that substantially any transmission line may be used in lieuof coaxial cable 430.

[0053] A flexible tubing 440, is effectively an outer sleeve that isformed over coaxial cable 430.

[0054] Typically, flexible tubing 440 may be made from any flexible,biocompatible material including, but not limited to, Teflon,polyethylene, and polyurethane. The thickness of flexible tubing 440 mayvary widely depending upon the requirements of a particular catheter. Byway of example, the thickness of flexible tubing 440 may vary betweenapproximately 0.005 inches and approximately 0.015 inches.

[0055] Electrode bands 444 are often “pressed into” flexible tubing 440such that electrode bands 444 may make contact with fluids and tissuethat are external to the catheter. In general, electrode bands areelectrically coupled to an external power supply (not shown) throughelectrode wires 448 which are located between flexible tubing 440 andco-axial cable 430. Electrode bands 444 may be used to monitorelectrocardiogram signals from a patient during an ablation procedure.As shown, electrode band 444 b, which is the electrode band which ismost distally positioned with respect to distal end 400 of catheter, issubstantially electrically coupled to outer conductor 436 through wires462. Such a connection to outer conductor 436 is generally made as closeto the distal end of outer conductor 436 as possible, as will beunderstood by those skilled in the art.

[0056] In one embodiment, electrode bands 444 may each have a width ofapproximately 0.004 inches, or approximately 1 millimeter, although thewidth of each electrode band 444 may vary. As previously mentioned,electrode bands 444 may be formed from substantially any suitablebiocompatible, material including, but not limited to, stainless steeland iridium platinum. Typically, the location of electrode bands 444 issuch that electrode bands 444 are relatively close to monopole antenna402.

[0057] A plug 460, which is formed around body section 408 andintermediate section 410 of monopole antenna 402, is arranged to holdmonopole antenna 402 with respect to flexible tubing 440. Such a plugmay be molded around at least a portion of monopole antenna 402 in orderto hold monopole antenna 402. As discussed above, plug 460 may be formedfrom any suitable, preferably biocompatible, material, which is capableof withstanding electromagnetic fields that may be produced usingmonopole antenna 402. By way of example, plug 460 may be formed from amaterial such as Teflon or polyethylene. The configuration ofintermediate section 410, with respect to body section 408 and tipsection 414, is arranged to hold plug 460 securely in place with respectto monopole antenna 402.

[0058] Although only a few embodiments of the present invention havebeen described, it should be understood that the present invention maybe embodied in many other specific forms without departing from thespirit or scope of the present invention. By way of example, an ablationcatheter that includes a monopole antenna which generates asubstantially deep electric field with respect to the monopole antennahas been generally described as being a microwave ablation catheter.However, such a monopole antenna may be use with various other cathetersincluding, but not limited, to catheters which operate using radiofrequency waves.

[0059] While a monopole antenna has been described as being formed froma material such as stainless steel, it should be appreciated thatmaterials used in the fabrication of a monopole antenna may vary widely.In general, monopole antenna may be formed from substantially anymaterial having a good electrical conductivity.

[0060] The sections of a monopole antenna, namely, the tip section, theintermediate section, and the body section, may take on various shapeswithout departing from the spirit or the scope of the present invention.By varying the shapes of the different sections, the shape of theelectric field which emanates from the monopole antenna may be varied.For example, in one embodiment, the body section of a monopole antennamay not have a tapered shape. In some cases, varying the shapesassociated with a monopole antenna may still enable the generatedelectric field to be substantially uniform. In other cases, varying theshapes may result in the generation of relatively non-uniform electricfields. The generation of relatively non-uniform electric fields may bedesirable, for instance, when a monopole antenna is to be used for anablation procedure that requires a specifically shaped electric field.That is, the tip section, the intermediate section, and the body sectionof a monopole antenna may be shaped to provide electric fields ofparticular shapes as required for specific ablation procedures.

[0061] A transmission line, e.g., the center conductor of a co-axialcable, has generally been described as being crimped, or otherwisecoupled, to the proximal end of a monopole antenna. It should beappreciated that a transmission line may be electrically coupled to themonopole antenna using various other methods, and at different locationswith respect to the monopole antenna. Therefore, the present examplesare to be considered as illustrative and not restrictive, and theinvention is not to be limited to the details given herein, but may bemodified within the scope of the appended claims.

What is claimed is:
 1. An antenna structure adapted to be used in anablation device, comprising: a monopole antenna operably disposed at adistal end of the ablation device and having a predetermined shapedefining an outer emission surface from which electromagnetic energy isemitted, the antenna forming the distal tip of the ablation device,wherein the predetermined shape of the antenna results in the creationof a relatively uniform electromagnetic field pattern.
 2. The antennastructure of claim 1, wherein the electromagnetic energy emitted issufficient to ablate biological tissue.
 3. The antenna structure ofclaim 1, wherein the antenna is encased in a biocompatible materialdefining an outer surface.
 4. The antenna structure of claim 3, whereinthe biocompatible material is Teflon.
 5. The antenna structure of claim1, wherein the antenna is formed from stainless steel.
 6. An ablationdevice for ablating biological tissue, comprising: an elongated flexibletubular member adapted to be inserted into a patient's body and having adistal end; a transmitting means operably attached to the tubular memberfor transmitting ablation energy therethrough; a monopole antennaattached to the distal end of the tubular member and having apredetermined shape defining an outer emission surface from whichelectromagnetic energy is emitted, the antenna forming the distal tip ofthe ablation device and operably attached to the transmitting means,wherein the predetermined shape of the antenna results in the creationof a relatively uniform electromagnetic field pattern.
 7. The ablationdevice of claim 6 further comprising a sensing means disposed on thedistal end of the tubular member proximal to the antenna for sensingelectro-physiological signals.
 8. The ablation device of claim 6,wherein the transmitting means is a transmission line.
 9. The ablationdevice of claim 8, wherein the transmission line is a coaxial cable. 10.The ablation device of claim 9, wherein the sensing means is at leastone electrode.